1. Field of the Invention
The present invention relates to an image data processing system and method.
2. Description of the Related Art
In codec standards for image data, such as moving picture experts group (MPEG)-2, MPEG-4, and H.264, an input image is divided into blocks having a predetermined pixel structure, e.g., 8×8 or 16×16 pixels, and discrete cosine transform (DCT) and quantization are performed on the divided image data for compression. The quantized image data is further compressed through entropy coding.
In general, image data are processed by an intra-compression method or an inter-compression method. The intra-compression method performs compression only using information within a single image frame. In image data, adjacent pixels are likely to have similar pixel values. The intra-compression uses such a feature to reduce the amount of data.
The inter-compression method encodes an image based on differences between pixel values of pixels between successive frames. Temporally successive images mainly include a motion of a person or an object only in the center of a screen without a change in the background of the person or the object, and temporal redundancy can be removed using this feature. In other words, a portion of a current frame, which is not changed from or is almost similar to a preceding or following frame, refers to the preceding or following frame without being encoded, thereby largely reducing the amount of compressed data.
Since the inter-compression method refers to a preceding or following image to generate a current image, the preceding or following image should be previously stored. A position to be referred to in the preceding or following image may be changed by the current image. Thus, the storage format of the preceding or following image should be such that it can be randomly accessed.
According to a block-based compression method that divides a frame into a plurality of blocks, the whole frame is divided into several blocks of a predetermined size and the divided blocks are independently compressed. FIG. 1 is a view for explaining compression of a frame in units of a block according to the Related Art.
As illustrated in FIG. 1, a frame is divided into blocks, each having 8*2 pixels, and each of the blocks is compressed. Each compressed block is stored in a designated buffer space of a memory unit (frame buffer) as a code. In FIG. 1, the designated buffer space for storing a single block is composed of 12 bits. U.S. Pat. No. 6,693,961 discloses a data processing technique for dividing a frame into a plurality of blocks and compressing the blocks.
The size of a designated buffer space occupied by a single block is determined according to a target compression rate and the designated buffer space is exclusively assigned to each block, thereby automatically determining the start position of each block. Since the start position of each block can be calculated through a simple operation, a plurality of blocks can be randomly accessed in hardware.
Various image data compression methods have been developed, but they do not usually support random accesses in a frame or are not suitable for actual application due to severe degradation in image quality even when supporting the random accesses. In other words, efficient image data compression should primarily aim at preventing degradation in image quality. However, the size of a compressed image and a memory capacity for storing the compressed image should also be considered, because they have a significant influence upon the cost of an apparatus for reproducing image data.
For example, to reproduce compressed data according to the compression standards stated above, a memory device having large capacity is required. A memory storage space of about 1.2 Mbytes is required to decode a PAL format used in common broadcasting or digital versatile discs (DVDs). However, in recently released decoding equipment, the entire system shares a 2-Mbyte memory device. As a result, 1.2 Mbytes occupy a large space of the 2-Mbyte memory device. This means that memory device should be used through efficient compression of image data. However, as mentioned above, it is a problem to handle image quality degradation or an increase in the size of compressed image data. In particular, in the block-based compression method, a compression failure occurs when the size of compressed image data exceeds the size of a compression space for each block.